The present invention relates to a method for making flame retardant polycarbonate copolymers having condensed polyhalo aromatic carbonate units. More particularly, the present invention relates to the phosgenation of bisphenol mixtures having up to about 50 mole % of polyhalo substituted bisphenol in the presence of an effective amount of a mixture of a tertiary organic amine and a phase transfer catalyst.
As shown by U.S. Pat. No. 3,912,687, tetrahalobisphenols can be polymerized under interfacial reaction conditions using relatively large amounts of trialkylamine catalyst in a two-stage reactor. However, a relatively low molecular weight polycarbonate is often formed. Polyhalobisphenol polycarbonates having a higher molecular weight can be made by the procedure shown by Ho et al, U.S. Pat. No. 4,794,156. Phosgene is added to an alkali metal polyhalobisphenate salt to produce the corresponding chloroformate. An activated pyridine, such as 4-dimethylaminopyridine, or 2-substituted pyridine is then added to increase the molecular weight of the resulting polycarbonate.
A method for making block copolycarbonates from diphenols and tetrahalogenated diphenols by phosgenation under interfacial reaction conditions is shown by Marks, U.S. Pat. No. 4,902,758. A mixture of bisphenol and a tetrahalogenated bisphenol are phosgenated under interfacial reaction conditions at different pH's to form bisphenol polycarbonate oligomers having chloroformate end groups and tetrahalogenated bisphenol chloroformate monomers. During a subsequent step, the chloroformate oligomers and monomers are coupled utilizing an activated pyridine catalyst. Another procedure for making copolymers of bisphenol A and a halogenated bisphenol utilizing a pentahalogenophenol as a molecular weight modifier, is shown by Komatsu et al, U.S. Pat. No. 4,918,155.
Although various procedures have been employed to phosgenate polyhalobisphenols and mixtures of polyhalobisphenols and bisphenols substantially free of aromatic ring substituted halogen groups, the molecular weight of the resulting polycarbonates are often lower than expected, and the phosgene requirements are often excessively high. More efficient condensation catalysts are therefore constantly being evaluated to minimize phosgene usage while improving molecular weithts of polycarbonate having polyhalo carbonate units.